Microsoft published a new knowledge base article on April 1st and its not a joke:
FIX: You experience very slow performance on an initial request to an ASP.NET Web site after you change the App_Code folder, the bin folder, or the Global.asax file
I installed the fix and it seem to cut down 50% of the startup time when I recompile my project. Try it out yourself and let me know if it helps.
The magic switch is optimizeCompilations=”true”.
“When it is turned on, the compilation takes a less conservative approach. If you only change the App_Code folder, the bin folder, or the Global.asax file, only the affected files are recompiled. Previously compiled pages are not recompiled.”
1: <compilation defaultLanguage="c#" debug="true" optimizeCompilations="true">
The Windows Vista version can be downloaded directly from here (even though the title says its for another fix).